In the production of microelectronic devices, photo-lithographic methods are used to transfer patterns onto photosensitive substrates to produce integrated circuits. A device suitable for maskless photo-lithographic pattern transfer is a “digital mirror device” (DMD), e.g. from Texas Instruments. A DMD comprises a micro-mirror array having about 1 million individually adjustable mirror elements. Tilting the mirror elements of the mirror array produces a pattern of radiating and not radiating mirror elements, which pattern is imaged with a projection optics or ray trap means onto the photosensitive substrate. Under computer control, various patterns can be produced and be maskless photo-lithographically transferred to the substrate. With a reproduction ratio of 1:1, a substrate surface of ˜10×14 mm is exposed.
The state of the art in the use of DMD's for the maskless lithographic transfer uses two principle methods in order to expose larger substrate surfaces: (1) the static step-and-repeat method, and (2) the scrolling method. The step- and repeat method (1) splits the picture information of the entire substrate into ˜10×14 mm partial fields, which are transferred consecutively with the illumination optics, with accurate edges onto the substrate. The continuous scrolling method (2) can be described as the exposure of a substrate surface element (pixel) by a mirror element. Mirror element and substrate surface element move relatively to each other with precisely controlled speed.
In order for the photolithographically produced substrate a surface element having the same length as the mirror element (with magnification 1:1), the relative movement may only have exactly one mirror element length. This condition is realized by the characteristic of the mirror array to be able to load new image information while the last image is still kept as pattern on the mirror elements. If due to this a pattern, set back by one mirror element, is loaded and switched through after a relative movement of a mirror element length of mirror array and substrate with respect to each other, a scrolling method develops, with which the mirror pattern appears to stand with respect to the substrate. However, a blur of pixel width develops at each edge.
Both methods have disadvantages. With the step-and-repeat method thousands of :-exact positionings have to be carried out, leading to more complex mechanics and to large dead times. The scrolling method accomplishes the uniform feed motion at the cost of “smeared” edge transitions and with a scan velocity limited by the mirror switching frequency, for example with a imaging ratio of 1:1˜135 mm/second. The mentioned scrolling methods additionally require a precisely controlled speed, thus inexpensive toothed belt drives are not usable. Accelerating and deceleration above the substrate during the exposure are not possible.
The state of the art is defined principally by U.S. Pat. No. 5,672,464, and U.S. Pub. Nos. 2005/0041229A, 2002/0012110A1, and 2005/0046819A1, the contents of which are incorporated by reference hereto and relied upon.